Pokemon Go actually supplies a couple of tool to help you find Pokemon but they are not very well explained. After a lot of trial error, reading and asking I have set upon a fairly solid way of triangulating a given Pokemon's location.

1) Leaves
When you are walking you will occasionally see leaves fly up in the air. This means a Pokemon is in that vicinity.2) The Sightings Screen - Nearby ScreenNote-this screen is now called Sightings after a Pokemon Go update.
The bar in the bottom right corner provides a screen that shows Pokemon nearby. This is the primary tool to locate Pokemon as you are walking around. It is also generally misused. When you open the Sighting- Nearby tool you see all the pokemon near you. You can see all the Pokemon that are in your general area. If you have the caught that type of Pokemon before it will show up in color, if not you will simply see a silhouette.

Steps:
Update 7 31, 2016 the Pokemon Go has received an update and the step icons have been removed. This is due to the steps not working correctly. I am going to leave the next bit on the steps just in case they are brought back.

The new Sightings screen does not seem to have a distance measure like the original footsteps. Instead all the pokemon appear in grass. The Sightings screen seems to have a smaller radius, and actively updates. When it pulses it is updating spawns and despawns.
The Sightings screen only shows a type of Pokemon once. Even if there are 5 ratatta only one will appear. This is so you can focus on the harder to find Pokemon.

Original Post
Each one has a number of steps underneath it. The steps measure the distance from and each Pokemon.

Steps:
3 Steps - Far away
2 Steps - Pretty Close
1 Step - Really Close
0 Steps - You are right on top of it just wait

More often than not you will turn this on and see that all of the Pokemon are 3 steps away.

3) Triangulation
The Steps do not tell you where the Pokemon actually are, or even in which direction they are from you. You can however use one extra piece of information to help with this. The Pokemon that appear are in an order where those in the top left are those that are closest to you. Those in the nearby screen toward the bottom right are the farthest from you. You can use this fact to triangulate upon the location of a Pokemon and at least get to the point where you hit 2 steps so you find a Pokemon's general location.

To do this try to go to an open area with lots of Pokemon. Then walk in a given direction while watching the Nearby screen. When a pokemon moves up in the order you are walking toward it, when it moves downward in the ranking you are moving away from it. If the rankings do not move then they could be clustered together or you should try to walk in an altogether different direction.

Sometimes the screen will freeze. When this happens I reset the game to ensure I have the freshest information.

Generally, the Pokemon do not seem to move that much. They seem to pop into existence at a given location so you have time to actively search for them.

The Pokemon that comes from Incense and Lures do not seem to be related to the Pokemon in the Nearby window. These Pokemon are separate from these.

4) The Perfect Spot
I find the ideal location to triangulate are open parks, fields where you can walk in 360 degrees. I prefer to start in a central area that gives me plenty of room to walk this way or that. Also make sure your phone is well charged or have a back up battery.

One other area that is a bit creepy but tends to be great in this game is cemeteries. For whatever reason there are a ton of Poke Stops in cemetaries and they are also fairly wide open to allow you to move around and triangulate upon where the Pokemon you are hunting are located.5) Learn the Spawn Spots
Many Pokemon seem to respawn after a given amount of time in a single spot. By recalling where you caught certain Pokemon you can return to pick up others often at the same locale.

6) Flashes - The nearby screen will occasionally Flash this means that the nearby screen has been updated. Check to see if any pokemon have altered in distance from you.